I've been in the same boat as you are last year and went for a mint green pick guard. That's the best choice if you are looking for that "50 years old UV aged" color. I had a few at the time, and the best was the Fender one (I don't remember where the other 2 were coming from but they were actually green, whereas the Fender really is a nice aged white).

Here's what I have....the pickup w/Aged White cover, a bright white Allparts pg, and a Fender '62 reissue white guard that seems to be darker. That one matches better, and I may go with it. I was just wondering if parchment or mint green would be even closer to matching the pickup cover. It is more cream-colored-

I've bought aged white covers from Fender before. Unless something has changed recently, they are indistinguishable from the parchment. I have a brand new, in the bag Fender parchment guard w. back plate if you want it. $15 shipped.

Fender American Standards Stratocasters come with a parchment pickguard and aged white pickup covers and knobs. Fender American Deluxe Stratocasters come with a mint pickguard and parchment pickup covers and knobs. FWIW, parchment covers and knobs are more yellow than aged white. With a white pickguard stick with either aged white or white.